Development of Novel Durable Solid Oxide Electrolysis Cells: Integration of Ni-GDC Fuel Electrodes into Fuel Electrode Supported Cells

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Abstract

Solid oxide electrolysis cells are attractive for power-to-X applications because they have unparalleled efficiency and can be thermally integrated with subsequent exothermic chemical synthesis processes further boosting efficiency [1], [2].

In this work a fuel electrode made from Ni and Gd-doped ceria (Ni-GDC) was integrated into a fuel electrode supported solid oxide electrolysis cell with a zirconia-based electrolyte. This type of solid oxide cell typically has a fuel electrode made from Ni and yttria-stabilized zirconia (Ni-YSZ), which is prone to degradation when operated at high current density. The aim of trying to integrate Ni-GDC was to improve durability when exposed to high current density. Previous work by Lenser et al. has also reported an attempt to make such a cell, but they saw a ∼50 pct drop in intial cell performance compared to Ni-8YSZ which was proposed to be related to interdiffusion between GDC in the fuel electrode and YSZ in the electrolyte [3].

The novelty of the approach pursued in this work was to reduce the sintering temperature to 1250°C in order to reduce interdiffusion. The developed fuel electrode supported cells were investigated alongside electrolyte supported cells with Ni-GDC electrodes to try and answer the following questions: Is reduction of the sintering temperature a viable solution to reach performance comparable to state-of-art Ni-YSZ based cells? Does the chemical expansion of GDC in fuel electrode conditions pose a critical mechanical threat to fuel electrode supported cells with a thin zirconia-based electrolyte? Does Ni-GDC also suffer from fuel electrode degradation at high current density when operated at 750°C.

The extent of interdiffusion was reduced by lowering the sintering temperature, but the cells produced in this work also suffered a ∼50 pct drop in performance compared to state-of-art Ni-YSZ based cells. Through detailed analysis of the cells using microscopy and electrochemical impedance spectroscopy it was concluded that the poor performance of the cells was mainly caused by poor contact between the electrolyte and GDC in the fuel electrode. This impacted most severely the resistance related to ionic transport across the fuel electrode-electrolyte interface. It was predicted that a cell with optimized processing would have a resistance contribution from ionic transport across this interface of 0.04 Ohm.cm2 at 800°C.

The mechanical robustness of the developed cells was investigated by operating a cell at very high cell voltage (> 2200 mV) and current density (-1.75 A/cm2). This did not cause cracks in the electrolyte, and it was concluded that chemical expansion is not a critical threat to cell integrity. The mechanics of GDC in reducing conditions were further investigated, and surprisingly creep was measured in constrained porous GDC. This means that internal stresses in a cell caused by chemical expansion of GDC will not reach the values predicted by classical laminate theory and linear elasticity.

Early attempts were made to evaluate the fuel electrode stability of Ni-GDC operated at 750°C. Symmetrical cells were tested between ±0.5 and ±1.3 A/cm2 for 1000 h and the fuel electrode resistance (Rp) was found to degrade less with higher current density. A similar observation was made for the developed fuel electrode supported cells. None of the tested cells were observed to exhibit signs of Ni-migration. Ohmic resistance was however found to increase significantly in both the developed cells and electrolyte supported symmetrical cells. Future work is needed to identify the source of this degradation and how it can be avoided.

Overall these were encouraging results, demonstrating a scalable manufacturing route capable of producing fuel electrode supported cells with a Ni-GDC fuel electrode. Work remains to be done in order to meet the predicted concept performance potential and to determine if the concept can meet expectations with respect to improved cell durability at high current density.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages144
Publication statusPublished - 2024

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